Add initial infrastructure for cursors. In where.c, optimize out clauses

of the form "ORDER BY rowid" if a table scan is being performed.  Do a
reverse table scan if "ORDER BY rowid DESC" is present. (CVS 2141)

FossilOrigin-Name: fc8c1393c86017a816beb52725b68af3b973f979

main.mk

@@ -54,7 +54,7 @@ TCCX = $(TCC) $(OPTS) $(THREADSAFE) $(USLEEP) -I. -I$(TOP)/src
 
 # Object files for the SQLite library.
 #
-LIBOBJ+= attach.o auth.o btree.o build.o date.o delete.o \
+LIBOBJ+= attach.o auth.o btree.o build.o cursor.o date.o delete.o \
          expr.o func.o hash.o insert.o \
          main.o opcodes.o os_mac.o os_unix.o os_win.o \
          pager.o parse.o pragma.o printf.o random.o \
@@ -71,6 +71,7 @@ SRC = \
   $(TOP)/src/btree.c \
   $(TOP)/src/btree.h \
   $(TOP)/src/build.c \
+  $(TOP)/src/cursor.c \
   $(TOP)/src/date.c \
   $(TOP)/src/delete.c \
   $(TOP)/src/expr.c \
@@ -226,6 +227,9 @@ config.h:
 	echo >>config.h
 	rm -f temp.c temp
 
+cursor.o:	$(TOP)/src/cursor.c $(HDR)
+	$(TCCX) -c $(TOP)/src/cursor.c
+
 date.o:	$(TOP)/src/date.c $(HDR)
 	$(TCCX) -c $(TOP)/src/date.c